The present invention relates to an ultrasound diagnostic apparatus and a method of producing an ultrasound image, and in particular, to an ultrasound diagnostic apparatus which produces an ultrasound image on the basis of reception data obtained by processing reception signals output from a transducer array having received an ultrasonic echo from a subject in a reception signal processor.
An ultrasound diagnostic apparatus using an ultrasound image has hitherto been put into practical use in the field of medicine. In general, in this type of ultrasound diagnostic apparatus, an ultrasonic beam is transmitted from a transducer array of an ultrasound probe toward a subject, an ultrasonic echo from the subject is received by the transducer array, and the reception signals are electrically processed in an apparatus body to produce an ultrasound image.
The ultrasonic beam is transmitted from a transducer array with a transmission focus position being defined on each scanning line. While in the vicinity of the transmission focus position, an image with high resolution is obtained because the ultrasonic beam is converged, at a position away from the transmission focus position, the ultrasonic beam is not sufficiently converged, and accordingly, the resolution tends to be lowered.
In view of the above, for example, as in an ultrasound diagnostic apparatus disclosed in JP 2009-22656 A, a technique in which a plurality of transmission focus positions are set in the measurement depth direction of each scanning line by an operator to perform multistage focus transmission is known.
If such a multistage focus transmission is used, the focus of the ultrasonic beam can be narrowed down over a wide range from a region where the measurement depth is small to a region where the measurement depth is large, making it possible to produce a high-quality ultrasound image.
However, if the multistage focus positions on each scanning line set by the operator are excessively close to each other, sound pressure in the vicinity of the focus position increases extremely, and significant irregularity occurs in amplitude of the reception signal, making it difficult to improve image quality over the entire image. The effect produced by closeness of a plurality of focus positions is especially great in a region where the measurement depth is small with little attenuation of ultrasonic waves.
On the contrary, in a region where the measurement depth is large, since attenuation of ultrasonic waves is large, even if the inter-focus position distance at the multistage focus positions is short, irregularity which occurs in the amplitude of the reception signal decreases.